When it comes to the election of the president of the Board of Supervisors on Tuesday, it may be best to start with the logical outcome and work backward. Virtually everyone predicts that incumbent President David Chiu will return for a third term.

Although Chiu doesn't inspire anyone to storm the battlements, he's done a deft job of navigating the lefty-middle of San Francisco politics. He's been on the smart side of polarizing votes - against reinstating Sheriff Ross Mirkarimi, for example - and has picked the candidates to support with care.

But there are other scenarios:

-- Scott Wiener: The moderate policy wonk falls just short of the six votes he needs. Wiener was always a contender, with five probable votes in his pocket. But the narrow District Seven election results doomed his chances. Wiener backed F.X. Crowley, who came up 131 votes short of Norman Yee. With Wiener stuck at five votes, the only person who could put him into the presidency is ... Chiu. And that's not happening.

-- Jane Kim: She sounds like a long shot and was said not to be "actively campaigning." But that's because she doesn't have to. Chinatown powerbroker Rose Pak is aggressively making calls that could be summarized as "anyone but David Chiu." It is possible to see Kim getting four solid votes - David Campos, John Avalos, Eric Mar and herself - but then she will need some moderates. That's where she may wish she hadn't voted to reinstate Ross Mirkarimi, because by the time she cast her vote, Mirkarimi already had the votes to keep his job.

-- Malia Cohen: A true compromise choice. Campos is said to be whispering presidential thoughts in Cohen's ear. Of course, Campos has his own agenda, since both he and Chiu are eyeing Tom Ammiano's state Assembly seat in 2014. Campos would rather keep Chiu's resume as thin as possible.

Cohen is definitely in the middle, and if the progressives want to make mischief, they could team up with some moderates - some of whom have already said they'd vote for her. That makes sense on paper, but Cohen has yet to show the political savvy needed to hold the job.

-- The end run: A January surprise is not impossible. Depending on how much they want Chiu to lose, progressives and moderates might consider pairing up to pick someone unexpected - say newcomer London Breed.

The problem is that the progressives would have to embrace a moderate - or moderates would have to get over their distaste for someone who voted to reinstate Mirkarimi. Neither seems likely.

On the other hand, back in 2009, a political newcomer came from out of nowhere to get the job as a compromise candidate. His name was David Chiu.